This patent specification relates to optical communications devices. More particularly, it relates to multiplexers/demultiplexers for use in wavelength division multiplexed (WDM) optical communications systems.
Wavelength division multiplexed (WDM) optical communication systems are based on the modulation of successive channels of information fi(t) onto successive optical carriers at wavelengths xcexi, which are then multiplexed onto a single fiber for transmission. In typical practical systems today, the bandwidth of each signal fi(t) may be about 10 GHz, the channel separation may be about 200 GHz (i.e. about 1.6 nm), and there may be about 20 channels being multiplexed onto the same fiber around a center wavelength of about 1540 nm. Thus, for example, a typical system may have 20 channels separately modulated onto carriers at 1530.0, 1531.6, 1533.2, 1536.8, . . . , 1560.4 nm, and the carriers are then optically combined into the same fiber for transmission. The above parameters are given by way of example only, the preferred embodiments described herein being applicable to any type of optical signal comprising a plurality of wavelength-division multiplexed signals at any of a variety of wavelengths.
Many useful devices in WDM communications systems are based upon WDM multiplexers/demultiplexers. Under the Principle of Reciprocity, most multiplexers are simply demultiplexers working in the opposite direction (and vice versa). Therefore, the preferred embodiments are described in terms of a demultiplexing function, it being understood that the preferred embodiments will operate as multiplexers in the opposite direction. The function of a demultiplexer is to receive a single optical beam carrying signals at xcex1xcex2xcex3xcex4 . . . xcexN and generate N separate beams, each carrying a different one of the optical signals xcex1, xcex2, xcex3, xcex4, . . . , or xcexN.
FIG. 1 illustrates a demultiplexer 100 according to the prior art, taken from Dutton, Understanding Optical Communications, Prentice-Hall (1998), which is incorporated by reference herein, in which narrow band transmissive-type dielectric thin film filters 102 are used. The thin film filters 102 are mounted on an SiO2 substrate 106, with GRIN lenses 104 being used to collimate the optical beam between free space and optical fibers as necessary. For the wavelength ranges of interest, each thin film filter 102 is designed to reflect all wavelengths of light except a single wavelength xcexi, with each filter being tuned to its own distinct wavelength xcexi. After a first wavelength xcex1 is extracted at the first filter, the remaining wavelengths xcex2xcex3xcex4 . . . xcexN are sent on to the next filter. The next filter extracts xcex2, and the remaining wavelengths xcex3xcex4 . . . xcexN are sent on to the next filter, and so on. It is to be appreciated that the demultiplexer 100 also operates as a multiplexer when operated in the reverse direction, and that only the demultiplexing direction is illustrated in FIG. 1 for clarity of presentation.
However, the use of transmissive type filters in a WDM demultiplexer results in difficulty in alignment, which is a major disadvantage. When the filters are transmissive, the xe2x80x9cbackendxe2x80x9d wavelengths near xcexN are inevitably reflected a large number of times before being directed to their final destinations. A misalignment xcex94xcex8 of any reflecting surface along the way causes a 2xcex94xcex8 error in the trajectory of the reflected light beam from that surface onward. Even if every subsequent filter was perfectly aligned, the divergence of the beam from the intended path is equal to 2xcex94xcex8 (in radians) times the distance traveled to the final destination. This error can become even worse if one or more subsequent filters is also misaligned. Thus, even a small angular error in any reflecting surface can cause severe system performance degradation or even system failure. Because of this, every reflecting surface needs special care during fabrication and assembly. This drives up the cost of components and assembly.
It should be noted that Dutton, supra in FIG. 1, presents one method of dealing with the alignment, which is to use a precisely cut SiO2 slab 106 as a substrate, carefully cut along the crystal axes so that the dielectric filters are precisely aligned. However, even this solution can be expensive, especially where cost savings are desired in as many aspects of a final product as possible. It would be desirable to use a less expensive material, such as plastic or standard glass, to hold the thin film filters. However, precise alignment using such low-cost materials would be very difficult, especially in view of their thermal sensitivity which can change the relative alignments in the event of uneven temperature distributions during the molding process or in field use.
Accordingly, it would be desirable to provide a WDM demultiplexer/multiplexer architecture that is more robust to small variations in the alignment of the channel filters.
A WDM demultiplexer/multiplexer is provided comprising a plurality of narrow band reflective filters linearly disposed along an optical axis, each narrow band reflective filter reflecting a single channel or group of channels and transmitting the remaining channels. The narrow band reflective filters are each tilted with respect to the optical axis. In a demultiplexing mode, an optical signal initially carrying channels at xcex1xcex2 . . . xcexN travels along the optical axis. Each narrow band reflective filter reflects a distinct one xcexi of the channels, directing the reflected beam away from the optical axis at twice the tilt angle toward an output. Each narrow band reflective filter is substantially transparent to the remaining channels of the optical signal, such that the remainder of the optical signal proceeds along the optical axis substantially undisturbed. Advantageously, the device is highly robust against tilt variations or other mechanical variations in the narrow band reflective filters, because such variations are not compounded as the optical signal travels through the device.
In a multiplexing mode, a plurality of optical signals xcex1, xcex2, . . . , xcexN are separately provided at the above outputs, and a multiplexed signal xcex1xcex2 . . . xcexN propagates out of the first narrow band reflective filter in a direction opposite the above optical signal. Preferably, the narrow band reflective filters are tilted less than 45 degrees with respect to the optical axis, and even better performance is achieved at less than 30 degrees. In one preferred embodiment, the narrow band reflective filters are dielectric thin film filters, while in another preferred embodiment they are holographic filters. A WDM demultiplexer/multiplexer in accordance with the preferred embodiments is readily adapted for use as a channel monitor and/or an add/drop multiplexer.
According to another preferred embodiment, when many channels xe2x80x9cNxe2x80x9d require multiplexing/demultiplexing, the incoming beam may be split into xe2x80x9cmxe2x80x9d separate beams and sent to xe2x80x9cmxe2x80x9d separate narrowband reflective filter arrays, each comprising about N/m narrowband reflective filters. Based on the number xe2x80x9cNxe2x80x9d and on system parameters such as beamsplitting loss and filter transmissivity, an optimal number for xe2x80x9cmxe2x80x9d may be determined based on a comparison of attenuation due to beam-splitting and attenuation caused by propagation through multiple serial filters.